Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has become one of the most powerful techniques for mass analysis and for the study of ion-molecule reactions.
A particularly useful technique is the Stored Waveform Inverse Fourier Transform (SWIFT) technique. In a SWIFT excitation experiment, the magnitude spectrum is specified by the user. A phase function is selected and the magnitude and phase functions are subjected to inverse Fourier transformation to produce the excitation waveform, which is then stored in a buffer memory. The stored digital waveform data are clocked out and these digital amplitudes are converted to an analog time domain analog excitation signal, amplified, and applied to the excitation plates of an FT-ICR cell. A particular frequency component of the excitation signal excites only those ions having a particular m/z ratio corresponding to the particular frequency component. Thus, by controlling the frequency and amplitude of the Fourier magnitudes included in the excitation signal, selected ions may be excited or ejected from the FT-ICR cell.
A major problem with the SWIFT technique has been the large excitation signal amplitude at the start of the excitation signal (t=0) attributed to the coherent summing of the various frequency components. This large signal amplitude often exceeds the dynamic range of the driver amplifier and is clipped thereby introducing spurious frequency components into the excitation signal and reducing the effectiveness of the SWIFT technique. Further, large digital amplitudes requires longer word length in the hardware, such as the analog to digital converter, for storing, transferring, and processing the time-domain data.
Various techniques have been developed to break the phase coherence at t=0. One technique is phase randomization. However, although this technique reduces the dynamic range of the excitation signal, phase discontinuity may cause the non-uniform excitation power observed between the specified inverse Fourier transform intervals in the resulting excitation signal. A second technique found useful for a uniform magnitude spectrum is a quadratic phase modulation technique. However, this is not a general method.
However, no effective phase function has previously been found to reduce the dynamic range of an excitation signal derived from a generalized magnitude spectrum.